Mineralogy of the Clay Fraction and Chemical Properties of Soils Developed from Sedimentary Lithologies of Piramb???3ia, Sanga-the-Cabral and Guar???? Geological Formations in Southern Brazil

摘要

ABSTRACT Quali-quantitative and crystallographic characterizations of the different minerals of a soil are essential for the best evaluation of its genesis, degree of weathering, active pedogenetic processes, physical and chemical properties, and its potential reserve of mineral nutrients. This study presents the results of mineralogical investigations carried out to verify the influence of the different lithologies of three geological formations of a sedimentary nature on the mineralogical composition of the soils and the relation of this mineralogical composition to soil chemical properties. The study area is located in the physiographic region of the Santa Maria River basin in the municipality of Ros????rio do Sul, RS, Brazil, at 30???° 15?￠???? 28?￠???? S and 54???° 54?￠???? 50?￠???? W, with average altitude of 132 m and Cfa climate type. Topolithosequences were defined from soils developed from the lithology of the Piramb???3ia, Sanga-do-Cabral and Guar???? Formations, choosing representative soil profiles based on types of parent materials and variations in relief and altitude. A general and morphological description of the profiles was carried out, and soil samples were collected for analysis. The properties determined were texture, active and potential acidity, organic C, exchangeable Al, sum of bases, CEC at pH7, and base saturation. Mineralogical characterization of the clay fraction was performed by an X-ray diffractometer (XRD). In most of the profiles, kaolinite was the predominant clay, along with a significant contribution from 2:1 clay minerals and the presence of interstratified minerals in combination with kaolinite, which were related to the chemical properties of the soils. The crystallographic parameters of the kaolinite (full width at half maximum - FWHM, layer-d spacing, mean crystallite size - D, and mean number of layers - NL) varied, revealing a distinct pedogenetic evolution, which can be attributed to different degrees of weathering, as well as differences in the source material. Large and asymmetrical reflexes related to the d001 and 002 planes of kaolinite indicated that they are due to the presence of kaolinite-smectite strata in association with pure kaolinites.